In this 1946 film, Holmes is hired by the Honorable Roland Carstairs (Geoffrey Steele) to guard the fabled 400-carat diamond "Star of Rhodesia," owned by Roland's mother, Lady Margaret (Mary Forbes). Believing that the gem will be stolen on a train trip from London to Edinburgh, Holmes secretly switches diamonds with Lady Margaret while in her compartment—just before Roland is killed and the fake diamond is stolen. Aided by Watson and Inspector Lastrade (Dennis Hoey), Holmes deduces which of the eccentric and suspicious passengers is the culprit.

Between 1939 and 1945, Universal Pictures filmed 14 Sherlock Holmes mysteries starring Philip St. John Basil Rathbone as Sir Arthur Conan Doyle's legendary detective and William Nigel Bruce as his faithful sidekick Dr. Watson. All but two of the films were "updated" to World War II England and often included Axis spies or saboteurs as antagonists. Each movie has been lovingly restored by the UCLA Theatre Archives from 35 mm master copies into a digital format and may be viewed in either colorized versions or in the original black and white.